Ruthenium Alloy Layer and Its Layer Combinations

ABSTRACT

Aqueous electrolyte for deposition of a ruthenium alloy layer on metal surfaces, in particular base metal surfaces, its use and a corresponding electrolytic process, and a correspondingly produced layer sequence.

The present invention is directed to an aqueous electrolyte for deposition of a ruthenium alloy layer on metal surfaces, in particular base metal surfaces. The present invention also relates to the use of the electrolyte according to the invention for producing ruthenium alloy layers on corresponding surfaces by means of an electrolytic process, which is also a subject matter of the invention. Finally, the invention also comprises layer sequences which have a ruthenium alloy layer deposited in this way.

Consumer goods and technical items, pieces of jewelry and decorative items are finished with thin oxidation-stable metal layers to protect them from corrosion and/or to enhance their appearance. These coatings must be mechanically stable and should not show any tarnish or signs of wear even after prolonged use. A proven means for producing such layers are galvanic processes, which can be used to obtain a wide variety of metal and alloy layers in high-quality form. Well-known examples from everyday life are galvanic bronze and brass layers on door handles or knobs, chrome coatings on vehicle parts, galvanized tools or gold coatings on watch straps.

In this context, galvanic palladium deposits and deposits of alloys of palladium have been known for a long time. Palladium is often used as a coating for, for example, electrical plug-in connections, printed circuit boards, decorative applications and many other industrial and commercial uses due to its bright color, excellent electrical properties, hardness, contact resistance and corrosion resistance as well as stability. As a substitute material for gold and platinum, palladium provided an economical, more cost-effective alternative because palladium also has a wide range of applications. Alloys of palladium with base metals such as nickel or cobalt are more cost-effective than the pure noble metal, so such alloys have long been used and continue to be used. Coatings of such palladium alloys are frequently produced by galvanic deposition from palladium alloy electrolytes.

Today, the price of palladium has soared to such an extreme that cheaper alternatives are being sought with which the use of palladium can be avoided while otherwise retaining the same properties. In this respect, the attention of a person skilled in the art is directed, among other things, to the use of substantially less expensive ruthenium.

The ruthenium baths and processes described in the prior art often refer to the deposition of black ruthenium and ruthenium alloy layers and may contain compounds of toxicological concern such as thio compounds as blackening additives (for example DE102011105207B4 and the publications cited therein). Often, due to their acidic character, these baths only allow deposition on metals which have a relatively noble character (for example DE1959907A1).

According to U.S. Pat. No. 4,082,625, bright ruthenium deposits can also be obtained in the basic range. Described is a process for deposition of ruthenium which operates in a pH range of 9-10. The ruthenium is kept in solution in this pH range by complexing anions (EDTA, NTA, CDTA). Stable and bright deposits of ruthenium are obtained.

A nitridochloro complex of ruthenium can be used in the aqueous non-acidic bath for electrodeposition of ruthenium. Such a method is described in U.S. Pat. No. 4,297,178. Also contained therein is oxalic acid or an oxalate anion. According to said method, only pure ruthenium deposits are generated; however, these deposits cannot replace palladium and palladium alloy deposits in this form without causing disadvantages.

Ruthenium deposits are mentioned, for example, in U.S. Pat. No. 3,692,641 or GB2101633. In the former, deposits of ruthenium with other noble metals, inter alia, are promoted. In the latter, ruthenium alloy deposits in acidic environments are addressed.

WO18142430A1 describes the production of different colored ruthenium or ruthenium alloy deposits with, inter alia, metals such as Ni, Co, Cu, Sn, etc. It is mentioned that deposits on base metal subsurfaces are possible. However, only strongly acidic electrolytes are presented here, which is why successful direct deposition on these subsurfaces is certainly not possible.

A way of successfully avoiding palladium-containing layers in electrolytic deposition practice is still being sought. For this purpose, the substitute layers should have properties as similar as possible to those of palladium-containing layers. This should apply in particular with regard to, inter alia, appearance, corrosion resistance, abrasion resistance and crack formation characteristics. It should further be possible to deposit appropriate layers on base metal surfaces in order to substitute the Pd-strike deposits frequently used for this purpose. In addition, substituting palladium should, of course, result in cost savings.

These and further objects which become apparent to a person skilled in the art are achieved by the disclosure of an electrolyte according to the present claim 1. Claims 2 to 7 are directed to preferred configurations of the electrolyte according to the invention. Claims 8, 12 and 15, respectively, are directed, together with the corresponding dependent claims, to the use of the electrolyte, a process for electrolytic deposition from the electrolyte, and a layer sequence obtainable therewith.

By using an aqueous electrolyte for deposition of ruthenium alloys on metal surfaces, in particular base metal surfaces comprising:

-   -   a) ruthenium as a bicyclic, anionic ruthenium nitrido complex         compound of the formula [Ru₂N(H₂O)₂X₈]³⁻, wherein X is one or         more singly or multiply negatively charged counterions, such as,         for example, halide ions, hydroxide ions or other anionic         ligands (e.g. sulfate, phosphate, oxalate, citrate), at a         concentration of 0.5-20 g/l based on ruthenium as metal;     -   b) one or more alloy metals dissolved in ionic form and selected         from the group consisting of: Cu, W, Fe, Co, Ni, In, Zn, Sn, Pd,         Pt, each at a concentration of 0.5-10 g/l based on the metal;     -   c) one or more anions of a di-, tri-, or tetracarboxylic acid at         a concentration of 0.05-2 mol per liter;     -   d) one or more anionic surfactants at a concentration of 0.1-500         mg/l; wherein the electrolyte indicates a pH of 5 to 10, the         solution to the objects presented above is surprisingly         achieved. The electrolyte presented here can be used to deposit         ruthenium alloy layers which are similar in color to the         palladium deposits and which have a high corrosion resistance,         low tendency to crack, and high hardness and thus high abrasion         resistance.

Ruthenium is preferably used in the form of a water-soluble compound known to a person skilled in the art as a bicyclic, anionic nitridohalogeno complex compound of the formula [Ru₂N(H₂O)₂X₈]³⁻, wherein X is a halide ion. The chlorocomplex [Ru₂N(H₂O)₂Cl₈]³⁻ is particularly preferred in this context. The amount of the complex compound in the electrolyte according to the invention can preferably be selected such that after the compound has been dissolved fully, the ruthenium concentration is between 1 and 20 grams per liter of electrolyte, calculated as ruthenium metal. The finished electrolyte particularly preferably contains 1 to 10 grams of ruthenium per liter of electrolyte, very particularly preferably 3 to 7 grams of ruthenium per liter of electrolyte.

The electrolyte contains certain organic compounds which have one or more carboxylic acid groups. In particular, these are di-, tri- or tetracarboxylic acids. These are well known to a person skilled in the art and can be found, for example, in the literature (Beyer-Walter, Lehrbuch der Organischen Chemie, 22nd Edition, S. Hirzel-Verlag, pp. 324 et seqq.). Particularly preferred in this context are acids selected from the group consisting of oxalic acid, citric acid, tartaric acid, succinic acid, maleic acid, glutaric acid, adipic acid, malonic acid, malic acid. Oxalic acid is particularly preferred in this context. The acids are naturally present in their anionic form in the electrolyte at the pH value to be set. The carboxylic acids mentioned here are added to the electrolyte at a concentration of 0.05-2 mol per liter, preferably 0.1-1 mol per liter and very particularly preferably between 0.2-0.5 mol per liter. This applies in particular to the use of oxalic acid, which is assumed to also serve as a conducting salt in the electrolyte.

In the electrolyte according to the invention, anionic surfactants are used as wetting agents. These are, for example, those selected from the group consisting of fatty alcohol sulfates, alkyl sulfates, alkyl sulfonates, aryl sulfonates, alkylaryl sulfonates, heteroaryl sulfates and salts thereof, and in particular alkoxylated derivatives thereof (see also: Kanani, N: Galvanotechnik; Hanser Verlag, Munich Vienna, 2000; pp. 84 et seqq.). Ethoxylated sodium fatty alcohol (C12-C14) ether sulfate or sodium fatty alcohol sulfate (C12-C14) are particularly preferred.

The pH value of the electrolyte is preferably in the only slightly acidic to slightly alkaline range. According to the invention, the pH value is set to a range between 5 and 10. More preferably, the pH value of the electrolyte is between 6 and 9 during use, particularly preferably between 7 and 8. Most preferably, a pH value of around 7.5 is set. The pH value during electrolysis is kept constant by adding buffer substances. These are well known to a person skilled in the art (Handbook of Chemistry and Physics, CRC Press, 66th Edition, D-144 et seqq.). Preferred buffer systems are borate, phosphate and carbonate buffers. Compounds for preparation of these buffer systems can be selected from the group consisting of boric acid, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium hydrogen carbonate or dipotassium carbonate. The buffer systems is used at a concentration of 0.08-1.15 mol per liter, preferably 0.15-0.65 mol per liter and very particularly preferably 0.2-0.4 mol per liter (based on the anion).

Of course, further substances advantageous for deposition can be added to the electrolyte discussed here. These are well known to a person skilled in the art. Preferred are those selected from the group consisting of conducting salts and brighteners, etc. (Praktische Galvanotechnik, 5th Edition, Eugen G. Leuze Verlag, Bad Saulgau, Germany, pp. 39 et seqq.). Preferred conducting salts are those selected from the group consisting of alkali sulfates, ammonium sulfate, ammonium chloride, ammonium oxalate.

In addition to the ruthenium, other dissolved metals are also present in the electrolyte. These are electrolytically deposited together with the ruthenium as a ruthenium alloy layer. Suitable are those selected from the group consisting of: Cu, W, Fe, Co, Ni, In, Zn, Sn, Pd, Pt. These are usually dissolved in the electrolyte as salts, in particular as sulfates. In this context, it is particularly preferred if the alloy metal is selected from the group of Ni, Sn, Zn, Co, Pd. Ni is most preferably used. The alloy metals are present in the electrolyte at a concentration of 0.1 to 10 g/l in each case. The concentration of the alloy metal is preferably 1-6 g/l and very particularly preferably 2-5 g/l. It has been found that adding the alloy metal to the electrolyte according to the invention in the specified concentration ranges helps to improve in particular the corrosion resistance and the tendency of the ruthenium layer to crack. Ni in particular has shown good results in this regard.

The present electrolyte may contain sulfur-containing compounds, for example the wetting agents or surfactants specified above. However, it is advantageous if the electrolyte does not contain sulfur-containing compounds in which the sulfur is present in an oxidation state of ≤+4. In particular, blackening additives based on sulfur compounds are not present in the electrolyte according to the invention.

The present electrolyte does not produce a black or dark anthracite-colored deposit, but rather a grayish, metallic-looking deposit. It thus resembles the Pd and Pd alloy deposits to be replaced, even in terms of appearance. The deposited alloy metal layer advantageously has an L* value of over 65. The a* value is preferably −3 to +3 and the b* value between −7 and +7, according to the Cielab color system (EN ISO 11664-4—latest version as of the filing date).

The present invention also relates to the use of the electrolyte just described for producing articles having an electrolytically deposited alloy metal layer on metal surfaces, in particular base metal surfaces, which comprise the metals ruthenium and one or more of the alloy metals dissolved in ionic form and selected from the group consisting of: Cu, W, Fe, Co, Ni, In, Zn, Sn, Pd, Pt, wherein the alloy metal layer has a corrosion resistance similar to that of the corresponding palladium-containing layers. Adding the listed metals also leads to a substantial reduction in the tendency to crack during electrolytic deposition compared with pure ruthenium deposits. The tendency to crack is preferably determined by visual inspection under an optical microscope at 20× magnification. Noble metal sublayers on which the ruthenium alloy layer can be deposited are known to a person skilled in the art. According to the invention, base metal surfaces are those which are unstable in an acidic or more basic environment and tend to dissolve. Preferred base metal layers for the ruthenium alloy layer are those selected from the group of copper, copper alloys, nickel or nickel alloys.

The ruthenium alloy layer obtainable by using the electrolyte according to the invention may have a certain thickness as specified by a person skilled in the art. The thickness of the alloy metal layer is preferably 0.05-5 μm, more preferably 0.05-2 μm and very preferably 0.05-1 μm. Further preferably, the alloy metal layer is used as a sublayer for a further metal layer to be electrolytically deposited, just as it is the case, for example, for Pd layers or Pd—Ni layers. The metal layer deposited on the ruthenium alloy layer can, for example, consist of noble metals such as, for example, Ag, Au, Pt, Rh or alloys thereof and generally has a thickness of 0.03-10 μm, preferably 0.05-3 μm and very preferably 0.1-1 μm.

It has been found that the metal deposits discussed here (for the ruthenium alloy layer itself as well as for the layer sequence) have a very high abrasion resistance, which is particularly advantageous both for the jewelry sector and for technical applications (for example, as a contact material). In the so-called Bosch-Weinmann test (Bosch-Weinmann, A. M. Erichsen GmbH, Publication 317/D-V/63, or Weinmann K., Farbe and Lack 65 (1959), pp. 647-651), the metal deposits of the ruthenium alloy with the electrolyte according to the invention achieve values of less than 0.25 μm/1000 strokes. Further advantageously, even less than 0.1 μm/1000 strokes and very advantageously less than 0.08 μm/1000 strokes are within the range of feasibility. For such abrasion-resistant metal deposits, the composition of the ruthenium alloy layer is very preferably 95:5 to 80:20, most preferably 90:10 to 80:20 based on the weight ratio of ruthenium to the other metal(s).

The present invention also relates to a process for deposition of an alloy metal layer on metal surfaces, in particular base metal surfaces, in which:

-   -   a) the metal surface is brought into contact as a cathode with         an aqueous electrolyte as just described;     -   b) an anode is brought into contact with the electrolyte;     -   c) and a sufficient current flow is established between the         cathode and the anode.

It should be noted that the embodiments described as preferable for the electrolyte and use thereof also apply mutatis mutandis to the process addressed here. The current density which is established in the electrolyte between the cathode and the anode during the deposition process can be selected by the person skilled in the art according to the efficiency and quality of deposition. Depending on the application and type of coating system, the current density in the electrolyte is advantageously set to 0.1 to 50 A/dm². If necessary, current densities can be increased or reduced by adjusting the system parameters, such as the design of the coating cell, flow rates, anode or cathode relationships, etc. A current density of 0.2-25 A/dm² is typically advantageous; 0.25-15 A/dm² is preferable, and 0.25-10 A/dm² is very particularly preferable. Most preferably, the current density is 0.25-5 A/dm². The selected value of the current density is also determined by the type of coating process. In a drum coating process, the preferred current density here is between 0.25 and 5 A/dm². In rack coating processes, a current density of 0.5 to 10 A/dm² leads to better results.

Typically, thin layer thicknesses in the range from 0.1 to 0.3 μm are produced in rack operation. Low current densities in the range from 0.25 to 5 A/dm² are advantageously used here. A further application of low current densities is used in drum or vibration technology, for example in the coating of contact pins. Here, approximately 0.25 to 0.5 μm thick layers are applied in the current density range of 0.25 to 0.75 A/dm². Layer thicknesses in the range from 0.1 to 1.0 μm are typically deposited in rack operation mainly for decorative applications with current densities in the range from 0.25 to 5 A/dm².

Instead of direct current, pulsed direct current can also be applied. The current flow is thereby interrupted for a certain period of time (pulse plating). In reverse pulse plating, the polarity of the electrodes is switched, such that the coating is partially detached anodically. By constantly alternating said anodic detachment with cathodic pulses, the build-up of the layer is thus con-trolled. Applying simple pulse conditions such as, for example, 1 s current flow (t_(on)) and 0.5 s pulse pause (t_(off)) at medium current densities leads to more homogeneous coatings (Pulse-Plating, J.-C. Puippe, F. Leaman, Eugen G. Leuzeverlag, Bad Saulgau, 1990).

When using the electrolyte according to the invention, insoluble anodes can preferably be used. Preferred as insoluble anodes are those made of a material selected from the group consisting of platinized titanium, graphite, mixed metal oxides, glass carbon anodes, and special carbon material (“diamond-like carbon,” DLC), or combinations of these anodes. Insoluble anodes of platinized titanium or titanium coated with mixed metal oxides are advantageous, wherein the mixed metal oxides are preferably selected from iridium oxide, ruthenium oxide, tantalum oxide and mixtures thereof. Iridium-transition metal mixed oxide anodes composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide, or iridium-tantalum mixed oxide are also advantageously used for execution of the invention. More information may be found in Cobley, A. J et al. (The use of insoluble anodes in acid sulphate copper electrodeposition solutions, Trans IMF, 2001,79(3), pp. 113 and 114).

The deposition of the ruthenium alloy layers on metal objects, in particular base metal objects, in accordance with the present invention can be carried out by way of example, taking the above into account, as follows:

To galvanically apply the ruthenium alloy layer, the pieces of jewelry, decorative items, consumer goods or technical objects to be coated (collectively referred to as substrates) are immersed in the electrolyte according to the invention. These form the cathode. An anode made of, for example, platinized titanium (product information—PLATINODE® from Umicore Galvanotechnik GmbH) is also immersed in the electrolyte. An appropriate current flow is then ensured between the anode and the cathode. A maximum current density of 10 amperes per square decimeter [A/dm²] has proven to be advantageous in order to obtain highly adhesive, uniform layers.

The temperature of the electrolyte during deposition can be set accordingly by a person skilled in the art. Advantageously, the temperature range to be set is from 20-80° C. Preferably, a temperature of 50° to 75° C. and particularly preferably 60° to 70° C. is set. It may be advantageous if the electrolyte under consideration is stirred during deposition.

Suitable substrate materials advantageously used here are copper base materials such as pure copper, brass or bronze, ferrous materials such as, for example, iron or stainless steel, nickel, gold and silver. The substrate materials may also be multilayer systems that have been galvanically coated or coated using other coating techniques. This applies, for example, to printed circuit board base material or ferrous materials that have been nickel-plated or copper-plated and then optionally gold-plated or coated with pre-silver. A further substrate material is, for example, a wax core which has been pre-coated with silver conductive lacquer (so-called electroforming).

A further subject matter of the present invention is a metal layer sequence comprising a substrate provided with a metal surface, in particular a base metal surface, an alloy metal layer which is electrolytically deposited thereon and produced by the process according to the invention and which has a thickness as described above, and a metal layer which is electrolytically deposited on said layer and is formed of noble metals such as, for example, Ag, Au, Pt or Rh and alloys thereof and has a thickness as also described above. The thicknesses of the layers can vary in the preferred ranges specified above. The preferred embodiments described for the electrolyte, its use and the process according to the invention also apply mutatis mutandis to the layer sequence described here.

The ruthenium alloy layer described here is an adequate substitute for the expensive Pd layers or Pd alloy layers, in particular Pd—Ni layers. Wherever the latter are advantageously used, the ruthenium alloy layer described here can be a more cost-effective alternative. In particular in the case of jewelry articles, a noble metal layer can be applied as a finish to the ruthenium alloy layer according to the invention. In particular, rhodium, platinum, gold and silver are suitable as noble metals. A person skilled in the art knows how to per-form such a finish.

However, the ruthenium alloy layer can also be established as a Pd or Pd—Ni substitute in articles used electronically. Here, rhodium. rhodium alloys for example RhRu), platinum, platinum alloys (PtRh, PtRu) or gold form preferred top layers. Thin palladium or palladium-nickel layers can also be applied as top layers. The top layer to be applied and its thickness depend on the intended application and are known to a person skilled in the art.

The present invention can preferably be used in drum and rack coating processes. Using the electrolyte described here, it is possible to achieve particularly crack-free, corrosion-resistant and abrasion-resistant deposits of ruthenium alloys on an appropriate substrate, which deposits are similar to Pd deposits. Furthermore, it is possible to work with the electrolyte in the neutral range, which for the first time allows ruthenium alloy coatings to be deposited on base metals without having to first provide the latter with a noble intermediate layer. In light of the known prior art, this was by no means obvious.

EXAMPLES

1 liter of the electrolyte specified in the respective exemplary embodiment are heated to the temperature specified in the exemplary embodiment by means of a magnetic stirrer, while being stirred with a cylindrical magnetic stirring rod 60 mm long at at least 200 rpm. This stirring and temperature is also maintained during the coating.

After the desired temperature has been reached, the pH value of the electrolyte is set using a KOH solution (c=0.5 g/ml) and sulfuric acid (c=25%) to the value specified in the exemplary embodiment.

Expanded metal sections made of platinized titanium serve as anodes.

A mechanically polished brass plate with a surface area of at least 0.2 dm² serves as cathode. This can be coated beforehand with at least 5 μm of nickel from an electrolyte which produces high-gloss layers. A gold layer approximately 0.1 μm thick may also be deposited on the nickel layer.

Prior to introduction into the electrolyte, these cathodes are cleaned with the aid of electrolytic degreasing (5-7 V) and an acid dip containing sulfuric acid (c=5% sulfuric acid). Between each cleaning step and before introduction into the electrolyte, the cathode is rinsed with deionized water.

The cathode is positioned in the electrolyte between the anodes and moved parallel thereto at at least 5 cm/second. The distance between anode and cathode should not change.

In the electrolyte, the cathode is coated by applying a direct electric current between anode and cathode. The current intensity is selected such that at least 0.5 A/dm² is achieved on the surface area. Higher current densities can be selected if the electrolyte specified in the application example is intended to produce layers that can be used for technical and decorative purposes.

The duration of the current flow is selected such that a layer thickness of at least 0.5 to 1 μm is achieved on average over the surface area. Higher layer thicknesses can be produced if the electrolyte specified in the application example is intended to produce layers of a quality that can be used for technical and decorative purposes.

After coating, the cathode is removed from the electrolyte and rinsed with deionized water.

The drying of the cathodes can take place via compressed air, hot air, or centrifugation.

The surface area of the cathode, the level and duration of the applied current, and the weight of the cathode before and after coating are documented and used to determine the average layer thickness as well as the efficiency of deposition.

Deposition of Ruthenium Alloy Layers

Example no. 1 2 3 4 5 Ru [mol/l] 0.05 0.05 0.08 0.02 0.05 Zn [mol/l] 0.003 — — — — Sn [mol/l] — — — 0.016 — Co [mol/l] — — — — 0.016 Ni [mol/l] — — 0.06 — — Pd [mol/l] — 0.0009 — — — Dipotassium oxalate [mol/l] 0.3 0.36 0.36 0.18 0.3 Tripotassium citrate [mol/l] 0.032 0.16 0.016 Ammonium sulfate [mol/l] 0.075 0.37 Dipotassium hydrogen — — — 0.43 — phosphate [mol/l] pH value 7.0 8.0 7.5 9.0 7.0 Temperature [C.] 65 60 65 55 65 Current density [A/dm²] 1 2 2 1 2 Alloy [wt. %] 60% 20% 10% 20% 15% Zn Pd Ni Sn Co Cracks none none none none none 

1-15. (canceled)
 16. An aqueous electrolyte for deposition of ruthenium alloys on metal surfaces, in particular base metal surfaces, comprising: a) ruthenium as a bicyclic, anionic ruthenium nitrido complex compound of the formula [Ru₂N(H₂O)₂X₈]³⁻, wherein X is one or more singly or multiply negatively charged counterions, at a concentration of 0.5-20 g/l based on ruthenium as metal; b) one or more alloy metals dissolved in ionic form and selected from the group consisting of: Cu, W, Fe, Co, Ni, In, Zn, Sn, Pd, and Pt, each at a concentration of 0.1-10 g/l based on the metal; c) one or more anions of a di-, tri-, or tetracarboxylic acid at a concentration of 0.05-2 mol per liter; d) one or more anionic surfactants at a concentration of 0.1-500 mg/1; wherein the electrolyte has a pH value of 5.0 to 10.0.
 17. The electrolyte according to claim 16, wherein the carboxylic acid is selected from the group consisting of oxalic acid, citric acid, tartaric acid, succinic acid, maleic acid, glutaric acid, adipic acid, malonic acid, and malic acid.
 18. The electrolyte according to claim 16, wherein the surfactant is selected from the group consisting of fatty alcohol sulfates, alkyl sulfates, alkyl sulfonates, aryl sulfonates, alkylaryl sulfonates, heteroaryl sulfates salts, and alkoxylated derivatives thereof.
 19. The electrolyte according to claim 16, wherein the pH value of the electrolyte is in a range of 7-8.
 20. The electrolyte according claim 16, wherein the electrolyte has a buffer system selected from the group consisting of borate, phosphate, and carbonate buffers.
 21. The electrolyte according claim 16, wherein the alloy metal is selected from the group of Ni, Pd, Pt, Sn, Zn, and Co.
 22. The electrolyte according claim 16, wherein it does not contain sulfur-containing compounds in which the sulfur is present in an oxidation state of ≤+4.
 23. A method for producing an article having a metal surface and an alloy metal layer thereon, which comprises electrolytically depositing the alloy metal layer on the metal surface using the aqueous electrolyte according to claim
 1. 24. The method according to claim 23, wherein the alloy metal layer has a thickness of 0.05-5 μm
 25. The method according to claim 23, wherein the alloy metal layer serves as a sublayer for a further electrolytically deposited metal layer of noble metals or alloys thereof, wherein the latter has a thickness of 0.05-5 μm.
 26. The method according to claim 23, wherein the metal layer(s) produced has/have an abrasion resistance of less than 0.25 μm/1000 strokes per the Bosch-Weinmann test.
 27. A process for electrolytically depositing an alloy metal layer on metal surfaces, in particular base metal surfaces, in which: a) the metal surface is brought into contact as a cathode with an aqueous electrolyte according to claim 16; b) an anode is brought into contact with the electrolyte; and c) a sufficient current flow is established between the cathode and the anode.
 28. The process according to claim 27, wherein the current density in the electrolyte is 0.1-50.0 A/dm².
 29. The process according to claim 27, wherein the temperature during the electrolysis is between 20° C. and 80° C.
 30. A metal layer sequence comprising a substrate provided with a metal surface, in particular a base metal surface, an alloy metal layer which is electrolytically deposited thereon and produced by a process according to claim
 27. 